Low-loss magnetic core of ferritic structure containing chromium

ABSTRACT

Corrosion-resistant magnetic core of low hysteresis loss and low eddy-current loss and alloy containing about 9% to 20% chromium, about 0.01% to 3% silicon and/or aluminum, manganese up to about 4%, carbon up to about 0.15%, about 0.15% to 1% sulfur and/or selenium, about 0.02% to 1% titanium and/or zirconium, and remainder substantially iron.

United States Patent .Patented Assignee Inventor Harry TanczynBaltimore, Md.

July 31, 1968 Oct. 26, 197 l Armco Steel Corporation Middletown, OhioAppl. No. Filed LOW-LOSS MAGNETIC CORE OF FERRITIC STRUCTURE CONTAININGCHROMIUM [56] References Cited UNITED STATES PATENTS 3,177,577 4/1965Fujimura 75/126 D 1,835,960 12/1931 Palmer 75/126 L 1,941,202 12/1933 DeFries 75/126L 3,362,813 1/1968 Ziolkowski 75/124 3,401,035 9/1968Moskowitz 75/124 Primary Examiner-l-Iyland Bizot Alt0rney-John HowardJoynt ABSTRACT: Corrosion-resistant magnetic core of low hysteresis lossand low eddy-current loss and alloy containing about 9% to 20% chromium,about 0.01% to 3% silicon and/or aluminum, manganese up to about 4%,carbon up to about 0.15%, about 0.15% to 1% sulfur and/or selenium,about 0.02% to 1% titanium and/or zirconium, and remainder substantiallyiron.

LOW-LOSS MAGNETIC CORE OF FERRITIC STRUCTURE CONTAINING CHROMIUM As amatter of introduction, my invention is concerned with a magnetic corefor electrical apparatus, and alloy steels.

One of the objects of the invention is the provision of a mag netic corepossessing a combination of good magnetic properties along with goodcorrosion resistance.

Another object is the provision of a magnetic core andcorrosion-resisting steel of high magnetic permeability, low reluctance,low hysteresis loss and low eddy-current loss, all at minimum cost, thatis, comparatively low cost in the ingot, in processing in the mill, andin use.

A further object of my invention is the provision of acorrosion-resisting alloy steel which is suited to the production ofbars, rod, wire and like mill products possessing a good combination ofmagnetic properties and machinability, that is, which readily lendsitself to cutting, threading, tapping, turning, milling and the like, asin the production of a variety of magnetic parts or components forelectrical machinery, apparatus and equipment.

A still further object is the provision of an alloy steel and variousflat-rolled products, such as sheet, strip, and the like, suited tovarious machining, forming and shaping, as in the production of thecores for electrical machinery and apparatus.

Other objects of my invention in part will become apparent and in partparticularly pointed to in the following description.

My invention resides in a magnetic core and steel, more particularly inthe combination of ingredients, making up the same, and in the relationbetween the ingredients, all as more particularly described herein, thescope of the application of which is set out in the claims at the end ofthis specification.

BACKGROUND OF THE INVENTION In order to gain a better understanding ofcertain features of my invention, it may be well to note at this pointthat the straight chromium grades of stainless steel have long beenaccepted in the art, many being identified by American Iron and SteelInstitute type numbers. For example, 1 refer to the more popular AlSlType 403 (11.5 to 13% chromium, carbon 0.15% max., manganese 1.00% max.,silicon 0.50% max., phosphorus 0.040% max., sulfur 0.030% max., andremainder iron); Type 405 (like Type 403 except 11.5 to 14.5% chromium,carbon 0.08% max., silicon 1.00% max. and 0.10 to 0.30% aluminum); Type414 (like Type 403 except 11.5 to 13.5% chromium, silicon 1.00% max.,and 1.25 to 2.50% nickel); and Type 416 (like Type 403 except 12 to 14%chromium, manganese 1.25% max., phosphorus 0.060% max., sulfur atleast0.15%, and silicon 1.00% max.

The steels noted, this with the exception of the Type 405 are suited toa variety of machined articles, parts and components. The steel of Type405, as distinguished from the others, is nonhardenable. it isparticularly suited for welded assemblies which are free of theair-hardening noted in the steels of Types 403 and 410.

Others of the commonly known and used straight chromium grades which arenot generally hardenable are the Type 430 (14 to 18% chromium, carbon0.12% max., manganese 1.00% max., phosphorus 0.040% max., sulfur 0.030%max., silicon 1.00% max. and remainder iron); Type 430F (generally likeType 430 but with sulfur at least 0.15%); Type 430FSe (similar to Type430 but with at least 0.15% selenium instead of the 0.15% sulfur); andType 442 (generally similar to Type 430 except 18 to 23% chromium andcarbon 0.20% max). These various steels are suited to a variety ofapplications where a nonhardenable corrosion-resisting steel isrequired; the steels containing a high sulfur and/or selenium contentare employed for a variety of machined articles and components.

While the straight chromium grades of stainless steel, as noted above,are suited to a wide variety of applications, none seems peculiarlyadapted to the use in electrical machinery, i.e., magnetic cores forsolenoid, relay and the like. Nor, indeed, for wider applications wherea controlled magnetic field of high permeance, low reluctance, and aminimum loss is desired. And while the steel of Type 4301- is readilymachinable and, as such, is suited to many applications, 1 findthat instraightening bar, rod, and wire stock, high stresses are developedwhich adversely affect the magnetic characteristics of the metal, aswell as the mechanical properties.

It is an object of my invention, therefore, to provide asteel whichenjoys a combination of corrosion resistance, good mechanical propertiesand good magnetic properties,narnely, high permeance, low hysteresisloss, and low eddy-current loss, all at minimum cost. 1

SUMMARY OF THE lNVENTlON hysteresis Referring now more particularly tothe practice of my invention, 1 provide a magnetic core and an alloysteel essentially consisting of about 9 to about 20% chromium(particularly about 12 to about 18% chromium), about 0.01 to about 3%silicon and/or aluminum (especially about 0.50 to about 2% silicon),about 0.15 to about 1% sulfur and/or selenium (especially about 0.15toabout 0.50 sulfur), about 0.02 to about 1% titanium and/or zirconium(particularly about 0.1 to about 0.6% titanium), and remaindersubstantially all iron. Carbon, of course, is present in my steel, thisin amounts up to about 0.15%, more particularly about 0.01% or even0.001%, to just under 0.06%, say to about 0.05%; for best results about0.01% to about 0.04%. Manganese, too, is present in my steel, this inamounts up to about 4%, more particularly about0.0l to about 1%. Theremainder of the steel, of course, is substantially all iron. The metalis not hardenable by heat-treatment; it is wholly ferritic with anabsence of austenite. And, of course, there is an absence of amartensitic constituent.

1 find that with with controlled carbon content and the essentialpresence of sulfur and titanium in the amounts indicated, the steel notonly is possessed of good mechanical properties with minimum adverseelTect resulting from straightening, bending, or the like, but that itis possessed of good magnetic properties. More particularly, the steelis of high magnetic permeance and of low loss, i.e., low hysteresis lossand low eddy-current loss. I attribute the superior magneticcharacteristics to a virtual freedom of the steel from intermetalliccompounds involving the iron present. In the prior corrosion-resistingsteels, I feel that certain compounds of iron, chromium, and carbon, andeven iron, chromium and nitrogen, are present which adversely affect themagnetic properties in that while readily magnetized, they are notreadily demagnetized. And, in a way, serving as permanent magnets asthey do, substantial loss is encountered with rapid reversal of themagnetic field as in alternating current electrical machinery, apparatusand equipment.

in the steel of my invention, I am inclined to the view that the carbonpresent in the metal appears in the form of titanium carbides, and thenitrogen as titanium nitrides, or perhaps other compounds involvingtitanium, carbon, nitrogen, and one or more of the alloying ingredientspresent, this excluding the iron, however. These compounds introduce nomagnetic effects, this because of the absence of iron in the compound.

Moreover, in my steel I feel that the eddy-current loss is effectivelyminimized as a result of the increase in the electrical resistance ofthe metal by reason of the chromium content, and, to some extent, thesilicon and aluminum contents. in consequence, the eddy-currents whichdevelop in the use of the metal as magnetic core for alternating currentelectrical applications are minimized by reason of the increasedelectrical resistance of the metal. Thus, there is enjoyed moreefficient operation with less heating in use.

in the steel of my invention, I feel that the desired magneticpermeability with low hysteresis loss :may be had with the use of one ormore of columbium, vanadium and molybdenum instead of the titaniumand/or zirconium addition. in general, however, I prefer the titaniumaddition for reasons of economy and for the further reason that itcombines with both the carbon and nitrogen contents of the steel,effectively eliminating the adverse effects of both.

In my steel the particular composition is considered to be in everysense critical. Although a rather wide latitude of chromium content iscontemplated, that is, from about 9 to about 20 percent, a steel withless than about 9 percent chromium is not acceptable because of a sharploss in corrosion resistance and, conversely, a steel having a chromiumcontent exceeding about 20 percent is not desired because of a sacrificein magnetic permeability. While the electrical resistivity of the metalincreases with the chromium addition, the permeability decreases. Forbest results a chromium content of about 12 to about 18 percent isdesired.

The ingredients silicon or aluminum generally are employed in my steelin small amounts, this not exceeding about 3 percent for the twotogether. These ingredients assure clean metal essentially free ofcontaminating oxide inclusions. A best steel employs silicon, this inthe amount of about 0.05 to about 2 percent, preferably about 1 to about2 percent for maximum cleanliness and an ease of fumacing, pouring andteeming. An excessive silicon content, however, indeed an excessivealuminum content, is not desired for it works adversely to the highmagnetic permeability which is sought. The same may be said with respectto the manganese content of the steel, since I view silicon, aluminumand manganese as additiveswhich are beneficial to the melting of thesteel, that is, in assuring an ease of fumacing and teeming with assuredcleanliness, but not beneficial to the desired magnetic properties.

The carbon content, the sulfur content, and the titanium content of mysteel, too, are viewed as critical, for with a carbon content exceedingabout 0.15 percent, the workability as by straightening, bending, andthe like, is inclined to suffer even though the machinability isimproved. A best combination of results is had where the carbon contentamounts to about 0.01 to about 0.04 percent, this assuring good bendingproperties, and a balance between an increase in the electricalresistivity resulting from the carbon addition and a decrease in themagnetic permeability. For some applications the carbon may range fromabout 0.01 percent to just short of 0.06 percent. For application wherethe metal is to be machined, the carbon content very well may approachthe 0.15 percent figure. On the other hand, where the metal is employedin the form of sheet, strip, or the like, and a deep-drawing operationis contemplated, the carbon content should be near the minimum figure,that is, about 0.02 percent or even about 0.01 percent.

Sulfur and/or seleniumin the amount of at least about 0.05 percent isfound necessary to achieve good machinability in my steel, while morethan 0.50 percent seems unnecessary. A sulfur and/or selenium contentexceeding about 1 percent is not acceptable, for I find with such a highcontent the workability in the hot-mill immediately suffers, withobjectionable tearing and splitting of the metal.

The importance of the titanium and/or zirconium addition, along with thesulfur and/or selenium and the chromium contents of the steel, isparticularly pointed to above. The amount of titanium and zirconium mustbe at least about 0.02 percent in order to enjoy any beneficial effect,but an amount exceeding about 1 percent not only produces no beneficialeffect, but represents an unnecessary cost. Moreover, the -excessivetitanium and/or zirconium in a measure detracts from the desiredmagnetic qualities of the metal. For best results it is titanium that isemployed, and this in the amount of about 0.1 to about 0.5 percent.

1 conveniently melt my steel in the electric arc furnace. Where desired,of course, it may be melted in the vacuum furnace. Or a combination ofarc-furnace melting and vacuumfumace refining may be employed toadvantage. But however melted, the steel handles well in the furnace, itteems well, and the molds strip from the ingots with ease.

The ingots in turn work well in the hot-mill at temperatures commonlyemployed. And so, too, the resulting blooms, billets, and the like, asin the production of plate, sheet, strip, bars, rods, wire, and specialshapes. Additionally, the metal works well in the cold-mill as in theproduction of cold-rolled DESCRIPTION-OF THE PREFERRED EMBODIMENTSWhile, as noted above, in broad aspect the magnetic core and alloy steelof my invention essentially consists of about 9 to about 20% chromium,about 0.01 to about 3% silicon and/or aluminum, manganese up to about4%, carbon up to about 0.15% (especially about 0.01% to just under0.06%), with about 0.15 to 1% sulfur and/or selenium (preferably about0.15 to about 0.50% sulfur), about 0.02% to about 1% remaindersubstantially all iron. Such a core and steel enjoys 1 an excellentcombination or magnetic permeability with low hysteresis loss, goodelectrical resistance and consequent low eddy-current losses, readymachinability, formability, and corrosion-resistance.

A further core and steel essentially consists of about 14 to about 16%chromium, about 0.5 to about 2% silicon, about 0.01 to less than 0.06%carbon (particularly about 0.01 to about 0.04% carbon), about 0.20 toabout 0.40% sulfur about 0.1% to about 0.6% titanium, and remaindersubstantially all iron. in this core and steel there is had a somewhathigher electrical resistance with consequently reduced eddy-currentlosses in use of the metal as a magnetic core component inelectricalmachinery, apparatus, and the like. The mechanical propertiesare good, and so, too, the magnetic permeability. And, too, thecorrosion resistance is somewhat improved, suiting it to someapplications in which the somewhat higher corrosion resistance is deemeddesirable.

A still further preferred core and steel enjoying a best combination ofcorrosion resistance, electrical resistance with minimum eddy-currentloss, along with good magnetic permeability, machinability, andmechanical properties, essentially consists of about 16 to about 19%chromium, about 0.5 to about 1% silicon, about 0.01 to about 0.05%carbon (particularly about 0.02 to about 0.04%), about 0.20 to about0.40% sulfur, about 0.1 to about 0.5% titanium, and remaindersubstantially all iron. Here, the lower maximum carbon content permitsthe lower maximum titanium content, with consequent savings and, yet,with a good combination of magnetic permeability, low eddy-currentlosses, and good corrosion resistance in magnetic core applications.

Another preferred core and steel essentially consists of about 12 toabout 18% chromium (more particularly about 14 to about 16% chromium),about 0.50 to about 3% silicon (especially about 1 to about 3% silicon),carbon less than 0.06% and preferably not exceeding 0.04%, about 0.15 toabout 0.50% selenium, about 0.1 to about 0.6% titanium, and remaindersubstantially all iron.

In specific illustration of the steels of my invention, 1 give TABLEI.-CHEMICAL COMPOSITION AND MAXIMUM PERMEABILIIY OFGHROMIUM-SILICON-SULPHUR- TITANIUM MAGNETIC CORE STEELS A review of thedata presented above rather clearly indicates that the maximumpermeability of the magnetic core steels of my invention progressivelyincreases with an increase in the chromium content and with an increasein the titanium content. Note, for example, that for the three steelsR5633-2, R5634-2 and R5635-2, having a titanium content of about 0.2%,the permeability for the steel of about 13% chromium amounts to 352,that for the 15% chromium steel comes to 546, and that for the 17%chromium steel amounts to 915. In these steel the silicon content isabout 0.30% and the carbon, manganese, phosphorus, sulfur and nickelcontents differ but little from one steel to the other.

For the three steels R5633-1, R5633-2 and R5633-3, with respectivetitanium contents of 0.01%, 0.21% and 0.37%, the maximum permeability isseen to be 237, 352 and 404, the permeability thus directly increasingwith the increase in titanium content.

To like extent, the permeability increases with respect to the siliconcontent, for it will be seen that the steel of about 13% chromiumcontent, 0.2% titanium content, with about 0.25% silicon, namelyR5633-2, has a permeability of 352, while the steel R5636-2, with likechromium and titanium contents but with a silicon content of 0.60%, hasa permeability of473. As the titanium content approaches 0.5%, however,and the chromium content is on the order of 17 little change inpermeability results from an increase in the silicon content, at thischromium level the permeability actually decreasing with an increase insilicon. Compare, for example, the 17% chromium, 0.46% titanium steelR5635-3, having a silicon content of 0.30% and a permeability of 1076,with the steel R5638-2 of about 17% chromium, 0.47% titanium and 0.71%silicon, having a permeability ofonly 908.

It is in the electrical resistivity, rather than the permeability, thatgreatest change is had with silicon content as seen from the datapresent in table 11 below. In that table there are given the electricalresistivities and the permeabilities in the annealed condition of twogroups of steels (and in the hot-forged condition for one group), onegroup of steel containing about 16% chromium and 0.2% titanium with thefurther ingredient selenium in the amount of about 0.15%, all ofdiffering silicon contents, and the other containing about 14% chromiumwith a sulfur content of about 0.3% and titanium content of about 0.15%,also of differing silicon contents. The electrical resistivity isexpressed in microohms per circular mill.

For the one group of magnetic core steels given in table II, this for achromium content of about 16 about 2% titanium and about 0.15% selenium,it will be immediately seen that the electrical resistivity, as well asthe permeability, increases with an increase in silicon content.Notably, with the increasing silicon contents of 0.48%, 0.92%, 1.89% and2.97% for the respective steels R6050, R6051, R6052 and R6053, theelectrical resistivities amount to 58.3, 72.5, 88.0 and 99.3 microohmsper circular mill. And for the group of steels of about 14% chromium,0.15% titanium, with sulfur rather than selenium, this in the amount ofabout 0.3%, the electrical resistivities amount to 67.5, 94.2 and 103.1microohms per circular mill for the three respective steels R6100, R6101and R6102, having silicon contents of 1.15%, 2.32% and 3.18%. Thepermeability of the steels of both groups increases with the increase insilicon content, going from 2194 in the annealed condition for the 16%chromium steel of 0.48% silicon content for the steel R6050 up to 3015for the steel R6053, with silicon content of 2.97%. The steels of thelower chromium content of 14% are possessed of significantly lowerpermeability but, here again, there is an increase in permeability withan increase in silicon content, the permeability increasing from steelR6100 having a silicon content of 1.15% and permeability of 1217, up to2628 for steel R6102 with silicon content of3.l8%.

In conclusion, it will be seen that I provide in my inventiona magneticcore and alloy steel in which the various objects hereinbefore set forthare successfully achieved. The core and steel are characterized bydesired magnetic permeability, particularly in the annealed condition,together with desired electrical resistivity. The steel, moreover, iscorrosion-resisting and well lends itself to a variety of machiningoperations, such as cutting, threading, tapping, turning, and the like,as in the production of magnetic cores for solenoid, relay and otherelectrical machinery, apparatus and equipment.

Inasmuch as many embodiments may be made of the core and steel of myinvention, and since many variations in the embodiments herein disclosedmay occur to those skilled in the art to which the invention relates, itwill be understood that all matter described herein is to be consideredillustrative and not by way of limitation.

I claim:

1. Magnetic core of ferritic structure and desired resistivity andpermeability for solenoid, relay or other electrical machinery,apparatus and equipment, said core essentially consisting of about 9 toabout 20% chromium, about 0.01 to about 3% silicon up to about about0.15% carbon, about 0.15 to about 0.50% ingredient of the group sulfurand selenium, about 0.02 to about 1% titanium, and remaindersubstantially all iron.

2. Magnetic core of ferritic structure and desired resistivity andpermeability for solenoid, relay or other electrical machinery apparatusand equipment, said core essentially consisting of about 12 to about 18%chromium, about 0.5 to about 2% silicon, up to about 4% manganese,carbon less than 0.06%, about 0.15 to about 0.50% sulfur about 0.1 toabout 0.6% titanium, and remainder substantially all iron.

3. Magnetic core of ferritic structure and desired resistivity andpermeability for solenoid, relay or other electrical TABLE II.CHEMICALCOMPOSITION, PERMEABILITY AND ELECTRICAL RESIS'IIVITY OF TWO GROUPS OFCIIROMIUM-SILICON-TITANIUM MAGNETIC CORE STEELS Maximum permeabilityElectrical re- Hot- 1450 F. sistance. micro- Heat No. 0 M11 P S Si Cr N1 Ti Se forged anneal ohm/cm.

machinery, apparatus and equipment, said core essentially consisting ofabout 12 to about 18% chromium, about 0.5 to about 2% silicon, about0.01 to about 0.04% carbon, about 0.15 to about 0.50% sulfur about 0.1to about 0.6% titanium, and remainder substantially all iron.

4. Alloy steel core offerritic structure and desired resistivity andpermeability for solenoid relay or other electrical machinery, apparatusand equipment, said core essentially consisting of about 12 to about 18%chromium, about 0.50 to about 3% silicon, carbon less than 0.06%, about0.15 to about 0.50%

2. Magnetic core of ferritic structure and desired resistivity andpermeability for solenoid, relay or other electrical machinery apparatusand equipment, said core essentially consisting of about 12 to about 18%chromium, about 0.5 to about 2% silicon, up to about 4% manganese,carbon less than 0.06%, about 0.15 to about 0.50% sulfur about 0.1 toabout 0.6% titanium, and remainder substantially all iron.
 3. Magneticcore of ferritic structure and desired resistivity and permeability forsolenoid, relay or other electrical machinery, apparatus and equipment,said core essentially consisting of about 12 to about 18% chromium,about 0.5 to about 2% silicon, about 0.01 to about 0.04% carbon, about0.15 to about 0.50% sulfur about 0.1 to about 0.6% titanium, andremainder substantially all iron.
 4. Alloy steel core of ferriticstructure and desired resistivity and permeability for solenoid relay orother electrical machinery, apparatus and equipment, said coreessentially consisting of about 12 to about 18% chromium, about 0.50 toabout 3% silicon, carbon less than 0.06%, about 0.15 to about 0.50%selenium, about 0.1 to about 0.6% titanium, and remainder substantiallyall iron.
 5. Alloy steel core of ferritic structure and desiredresistivity and permeability for solenoid, relay or other electricalmachinery, apparatus and equipment, said core essentially consisting ofabout14 to about 16% chromium, about 1 to about 3% silicon, carbon notexceeding 0.04%, about 0.15 to about 0.50% selenium, about 0.1 to about0.6% titanium, and remainder substantially all iron.